$global_bg_color: #ddd; //背景颜色
$global_box_shadow:0 2px 4px rgba(0, 0, 0, 0.12), 0 0 6px rgba(0, 0, 0, 0.04); //边框效果
@mixin global_Center{ //定位居中
  position: absolute;
  padding: 50px 20px;
  left: 50%;
  top: 50%;
  transform: translate(-50%,-50%);
}